1. Field of the Invention
This invention relates to a method of producing a silicone elastomeric foam from an aqueous emulsion of anionically stabilized, hydroxyl endblocked polydiorganosiloxane, colloidal silica, organic tin compound, and water.
2. Description of the Prior Art
A method of foam production from elastomeric emulsions is known as the Talalay process. As described in U.S. Pat. No. 2,432,353, issued Dec. 13, 1938, a natural rubber latex is compounded with curing agents, then caused to expand by mixing with hydrogen peroxide. While expanding, the mixture is poured into an aluminum mold. After the expanding mixture fills the mold, the contents of the mold are frozen by immersing the mold in a brine at -30.degree. C. The mold is then connected to a vacuum and a supply of alkaline calcium chloride brine at -2.degree. C. is drawn through the expanded, frozen mixture to cause an irreversible coagulation to take place. The mold is then transferred to a live steam vulcanizer where it is heated for 25 minutes at 125.degree. C. to vulcanize the natural rubber. The vulcanized foam article is then removed from the mold, washed, centrifugally extracted and dried. Talalay teaches the preliminary formation of the foam can be effected by any of the known procedures, such as mechanical whipping, chemical gas generation, or physical release of gas or vapor.
In the book, "Neoprene Latex", by J. C. Carl, published by E. I. Dupont DeNemours and Co. (Inc.) (1962) a process of converting specialized neoprene latex to foam is described. The latex fillers, curing agent, accelerator, and foam stabilizing surface active agents are mixed, then the mixture is stirred rapidly to whip in air and create a froth. The froth is stirred until it is refined to a smooth cream. A gelling agent is then added and the froth placed in a mold. The foam produced by the gelling of the froth is usually cured by exposure to steam. After curing, the foam is washed and dried.
The processes as discussed above require the gelling of the froth to stabilize the foam by the use of either an internal gelling agent as discussed for a neoprene latex or an external gelling agent as discussed with a natural rubber latex. This gelling step must be carefully controlled in order to produce satisfactory foam. Dunn in U.S. Pat. No. 3,491,033, issued Jan. 20, 1970, describes a process of making a solid cellular material that does not contain a gelling step. Dunn states that the process is applicable to any polymer that is available in the form of an aqueous emulsion and is film forming at a temperature below 300.degree. F. Dunn shows examples of foam production with natural rubber latex, butadiene-styrene latex, and also emulsions of thermoplastic polymers such as polyethylene and vinylchloride. His process is carried out by adding a foaming agent, incapable of gelling the wet foam, to an aqueous dispersion, forming the mixture into a wet foam, drying the wet foam into a solid cellular material, and heating the solid cellular material to the film-forming temperature of the polymer. Dunn uses relatively large amounts of foaming agents to allow the foam structure to retain its cellular shape during the drying step. The instant invention does not fall under Dunn as the emulsion cannot be dried to a solid cellular material as called out by Dunn without gelling the foam which is contrary to the requirement of Dunn.